Silk plants are generally preferred, but I personally use plastic just because for me they are cheaper, last longer, and my LFS is having a sale on them.... Just be extremely careful with plastic plants, make sure there are no sharp edges, my store lets me slide the back out of the packaging and feel the plants before I buy, ask if you can or if you can return them opened if they don't work and check in the parking lot (I do this at petsmart because I'm slightly paranoid of ripped fins, lol). I like plants that feel almost soft, to the touch, I just bought some Marina aquascaper plants that fit the bill pretty well. And of course, run a cotton ball over the plants to make sure nothing snags before adding them to the tank.

I got all my silk plants from Petco.com and in store. Sometimes the stores selection isn't great but the prices are usually better online and if you spend 50$+ you get free shipping. You can tell if it has metal in it usually by if it stands up on it's own sort of well. All mine just sort of flop over out of the water so pretty sure there's no metal.